Bypass the JQuery-Real-Person captcha plugin 0-day

# Exploit Title: Bypass the JQuery-Real-Person captcha plugin

# Google Dork: [if relevant]  (we will automatically add these to the GHDB)

# Date: 28th November, 2011

# Author: Alberto Garcia-Illera

# Software Link: http://keith-wood.name/realPerson.html

# Version: 1.0.1 (0-day)

# Tested on: Chrome web browser

# Vulnerable software can be downloaded from this link:
http://keith-wood.name/zip/jquery.realperson.package-1.0.1.zip

It is possible to bypass the captcha on the JQuery-Real-Person plugin to
perform a brute force attack.

There is associated parameter with each image, to checkout the characters
introduced by the user. But there is not a good chek to assure that the
characteres introduced are the characters shown on the picture.

Therefore we can just choose a pair of parameter and characters and use
them in all the request to the web server.

The name of the parameter that determinate the captcha image is "value".

Example: The captcha image shown in the example is JYYBME and we use
"Inspect Element" on Google Chorme or Firebug on Firefox to search this
line in the code:

<input type="hidden" class="realperson-hash" name="defaultRealHash" *
value="-1158072107"*>

In this case we already know a valid pair of parameter and characters that
we can use to perform a brute force attack bypassing the captcha
restriction.

JYYBME ----> *-1158072107*

We can generate as many valid pairs as we want but only one is necessary to
perform the brute force attack.

It does not matters that the captcha does not show the characters that we
type because the check is done through the value parameter so we just need
to type one valid pair of parameter and characters.

Thanks to Dani and Manu (The Sur) for helping me teaching me, every day, a
different knowledge.